Conventionally, nanowire arrays are grown on crystalline substrates with an epitaxial relationship such that the crystalline nanowires are generally vertically aligned due to the homo-epitaxy or hetero-epitaxy, or in some cases at angles with respect to the substrate.
A common technique for creating nanowire arrays is by a vapor-liquid-solid (VLS) synthesis process. This process uses as source material such as a feed vapor gas such as silane. The silane is then exposed to a catalyst such as liquid metal nanoparticles (e.g., gold), which are deposited on a substrate by evaporation or sputtering. The silane decomposes and dissolves into the nanoparticle, and when the silane reaches supersaturation in the metal, it precipitates out as a single crystal silicon wire. Templates having pores, such as an anodic aluminum oxide (AAO) templates, have been formed on the substrates to arrange and align the growth of the nanowires. Where the substrate is glass, a metal layer is typically disposed between the glass and the AAO template. However, a problem with the above technique is that upon exiting the AAO the wire is free to expand, plausibly causing catalyst transport on top of the template that can lead to secondary growth of nanowires from the surface of the AAO template or the merging of catalyst nanoparticles or droplets.
Lee et al., “Well-Ordered Co Nanowire Arrays For Aligned Carbon Nanotube Arrays”, Synthetic Metals, Volume 124, Number 2, 22 Oct. 2001, pp. 307-310, discloses a process of forming carbon nanotube arrays which includes an anodization process to form a plurality of pores in an AAO film, overdepositing cobalt catalyst in the pores, polishing the surface, and etching back the cobalt.
Lee et al., “Uniform Field Emission From Aligned Carbon Nanotubes Prepared By CO Disproportionation”, Journal of Applied Physics, Volume 92, Number 12, 15 Dec. 2002, pp. 7519-7522, discloses a process of forming carbon nanotube arrays which includes an anodization process to form a plurality of pores in a bulk AAO film, overdepositing cobalt catalyst near the mouth of the pores, and then a two step process one to etch back the overfilled cobalt catalyst and the other to widen the pores above the cobalt.
There is a need for further methods for forming nanostructures, and in particular, to methods for forming nanostructure arrays such as nanowire arrays.